Conventionally, a technique that determines an immediately front vehicle with respect to a host vehicle is disclosed in, for example, JP 2002-222491 A. In JP 2002-222491 A, an apparatus, which is equipped in the host vehicle, detects inter-vehicle distances to multiple front vehicles based on position information received from a global positioning system (GPS). At the same time, the apparatus detects a distance from the host vehicle to a front object based on a detection value received from a radar detector. The apparatus determines then one of the front vehicles as the immediately front vehicle when one of the detected inter-vehicle distances is equal to the distance from the host vehicle to the front object.
Further, a technique that determines an anteroposterior relation of vehicles traveling in a convoy is disclosed in, for example, JP H09-081899 A (corresponding to U.S. Pat. No. 5,680,122). In JP H09-081899 A, an apparatus changes an identification number from a leading vehicle in the convoy based on a predetermined rule, and transmits the identification number to an immediately following vehicle. Herein, the predetermined rule may be increasing a value of the identification number by one at one time. With this configuration, an order number of each vehicle included in the convoy can be identified.
The host vehicle in JP 2002-222491 A uses the radar detector. Thus, the host vehicle can specify only the immediately front vehicle and the immediately following vehicle. Further, in JP H09-081899 A, only the anteroposterior relation of the vehicles traveling in the convoy is determined.
Usually, there are many vehicles traveling on the road. Thus, not only a behavior of the immediately front vehicle, but also behaviors of front vehicles traveling in front of the immediately front vehicle are necessary to improve a safety of a driving. For example, when one of the front vehicles in front of the immediately front vehicle decelerates, a driver of the host vehicle may be informed the deceleration before the immediately front vehicle decelerates. Thus, the driver may promptly control the host vehicle to take a necessary action.
When the host vehicle is equipped with a vehicle-to-vehicle communication device, the driver can be informed of behavior information of the vehicles within a communication range of the communication device. When one of the front vehicles decelerates, a deceleration timing and a deceleration degree differs based on the anteroposterior relation between the host vehicle and the decelerated front vehicle. In order to perform a vehicle control with a high accuracy, the anteroposterior relation between a target vehicle and the host vehicle needs to be determined. Herein, the target vehicle is the vehicle from which the behavior information is received. The anteroposterior relation between the target vehicle and the host vehicle signifies an order number and a position direction of the target vehicle with respect to the host vehicle. Usually, the host vehicle receives behavior information from multiple vehicles through a vehicle-to-vehicle communication. Thus, identifying a subject vehicle of each behavior information is difficult.
Further, in JP 2002-222491 A, only the immediately front vehicle is specified. That is, other front vehicles and following vehicles apart from the host vehicle are not specified. In JP H09-081899 A, only the anteroposterior relation of the vehicles traveling in the convoy is specified.